Abnormal heart beats or cardiac arrhythmias can cause significant morbidity and mortality. These arrhythmias arise from a variety of causes, including atherosclerotic heart disease, ischemic heart disease, metabolic or hemodynamic derangements, rheumatic heart disease, cardiac valve disease, certain pulmonary disorders and congenital etiologies. The normal heart rate is about 60 to 100 beats per minute. Arrhythmias refer to tachycardias at rates exceeding 100 beats per minute for a duration of at least 3 beats. Sometimes no treatment is required, such as in the tachycardia following a physiologic response to stress or exercise. However, in some cases, treatment is required to alleviate symptoms or to prolong the patient's life expectancy.
A variety of treatment modalities exist, including electric direct current cardioversion, pharmacologic therapy with drugs such as quinidine, digitalis, and lidocaine, treatment of an underlying disorder such as a metabolic derangement, and ablation by either percutaneous (closed chest) or surgical (open chest) procedures. Treatment by ablation involves destruction of a portion of cardiac tissue which is functioning abnormally electrically.
Normally the heart possesses an intrinsic pacemaker function in the sinoatrial (SA) node which is in the right atrium, adjacent to the entrance of the superior vena cava. The right atrium is one of four anatomic chambers of the heart. The other chambers are the right ventricle, the left atrium, and the left ventricle. The superior vena cava is a major source of venous return to the heart. The SA node is an area of specialized cardiac tissue called Purkinje cells and which usually measures roughly 11/2 centimeters by about 21/2 millimeters. An electrical impulse normally exits from the SA node and travels across the atrium until it reaches the atrioventricular (AV) node. The AV node is located in the right atrium near the ventricle.
Emerging from the AV node is a specialized bundle of cardiac muscle cells which originate at the AV node in the interatrial septum. This "bundle of His" passes through the atrioventricular junction and later divides into left and right branches which supply the left and right ventricles. The left and right bundles further give rise to branches which become the so-called distal His-Purkinje system, which extends throughout both ventricles.
Thus in a normal situation an impulse originates intrinsically at the SA node, transmits through the atrium and is modified by the AV node. The AV node passes the modified impulse throughout the left and right ventricles via the His-Purkinje system to result in a coordinated heartbeat at a normal rate.
Many factors affect the heart rate in addition to the intrinsic conduction system. For example, normally the heart rate will respond to physiologic parameters such as stress, exercise, oxygen tension and vagal influences. Additionally, there are multiple causes for an abnormal heartbeat such as an abnormal tachycardia. One group of such causes relates to abnormalities in the heart's conduction system. For example, ectopic or abnormally positioned nodes may take over the normal function of a node such as the SA or AV node. Additionally, one of the normal nodes may be diseased such as from ischemic heart disease, coronary artery disease or congenital reasons. Similarly, a defect can exist in an important part of the conduction system such as the bundle of His or one of the bundle branches supplying the ventricles.
Treatment of abnormal tachycardias arising from ectopic foci or so-called ectopic pacemakers can include pharmacologic therapy or ablative therapy. The ablative therapy may be accomplished by percutaneous insertion of a catheter or by an open surgical cardiac procedure.
Cardiac arrhythmias may be abolished by ablating the tissue responsible for the genesis and perpetuation of the arrhythmias. Ablation catheters using radio frequency (RF) energy are known. Additionally ethanol has been infused into coronary arteries to ablate a focus such as a ventricular arrhythmia focus or the AV node. Unfortunately this tends to result in a fairly large region of cardiac tissue death or myocardial infarction. With transarterial infusion there is difficulty in precisely controlling the location and extent of the ablation.
During conventional ablation of cardiac tissue in attempts to ablate arythmogenic foci, such as the AV node, AV node reentry, Wolff-Parkinson-White, and ventricular tachycardia foci, it is difficult to know the size and extent of the lesion being created. With RF energy, electrode tip temperature serves as a rough guide that a lesion is being created and as to the size or volume of the lesion. However, variation in the angle of the tip with respect to the tissue and the rapidity of blood movement among other factors makes this a very rough, indirect guide at best.
Normally when performing a percutaneous or closed chest cardiac ablation procedure, fluoroscopy is used to visualize the chambers of the heart. Fluoroscopy uses roentgen rays (X-rays) and includes use of a specialized screen which projects the shadows of the X-rays passing through the heart. Injectable contrast agents to enhance the fluoroscopic picture are known.
Some of the problems with conventional fluoroscopic positioning of catheters include prolonged radiation exposure, sometimes as long as 2 hours. Additionally, the clinician may be unable to determine precisely where the catheter is in terms of the endocardium and cardiac structures, such as valves. Additionally, the tissue is not characterized before or after ablation except in electrical terms. The thickness or volume of the tissue is not assessed.
Thus, the prior art lacks methods and devices useful for precisely ablating cardiac tissue and accurately determining the extent of the lesion being created. The present invention addresses these and other needs.